Dry eye syndrome is one of the most commonly treated eye problems in the United States. Dry eye syndrome is also known as keratitis sicca, keratoconjunctivitis sicca (KCS) xerophthalmia, and lacrimal insufficiency. It is estimated that over ten million Americans and 30 million persons worldwide suffer from dry eye syndrome.
For a large fraction of dry eye patients, dry eye syndrome creates discomfort or annoyance. For those severely afflicted, dry eye syndrome can be debilitating and, in some circumstances, even sight-threatening. In extremely severe cases, dry eye syndrome can even lead to the loss of an eye.
Dry eye syndrome typically results from deficiency in the quality or quantity of tears produced by the patient. Precorneal tear film has traditionally been considered to have a three-layered structure. The closest to the cornea lies the mucin, or mucus, layer. The mucin layer provides an interface between the corneal epithelium and the remainder of the tear film. Overlying the mucin layer is the watery aqueous layer, which is the thickest layer of the three. The outermost layer of the precorneal tear film is the lipid layer. The lipid layer is an oily film that reduces evaporation from the aqueous layer beneath it.
The middle aqueous layer provides moisture to the corneal tissue, carries important nutrients, and serves to remove metabolic waste produced by the cornea. Deficiency in any of the three layers of the precorneal tear film can result in complaints of dry, gritty feeling or burning eyes.
The mucin that forms the mucin layer, nearest the cornea, is secreted by goblet cells in the conjunctiva. The conjunctiva is the transparent tissue that covers the sclera and the backside of the eyelids. The mucin layer functions to decrease surface tension of the tear film. In addition, the cornea itself is hydrophobic. Without the mucin layer to provide a bridge between the cornea and the aqueous layer, the aqueous layer would bead up and allow dry spot formation on the cornea.
The aqueous layer is secreted primarily by the glands of Wolfring and Krause located in the eyelid margin. The aqueous layer helps provide an optically smooth, transparent surface to the precorneal tear film. The lipid layer is secreted by the meibomian glands, and the glands of Zeiss and Moll. The glands of Zeiss and Moll are also located at the eyelid margin.
Blinking is essential to maintenance of the precorneal tear film. During each blink, the eyelid wipes over the surface of the cornea, smoothing the mucin layer and spreading the overlying aqueous and lipid layers to provide a completely wetted surface. In between blinks, the tear film thins due to evaporation of the aqueous layer. If evaporation is excessive, dry spots may form on the surface of the cornea.
Deficiency, or imperfect quality, of any of the three component layers can lead to dry eye symptoms. Many systemic and external factors can contribute to dry eye syndrome. For example, Sjogren's syndrome is associated with arthritic diseases in combination with dry eye and dry mouth. Deficiency of Vitamin A, use of oral contraceptives and environmental factors can all contribute to dry eye syndrome.
Recent research into the natural history of dry eye syndrome has shown that the disease progresses through four stages. Each stage is a consequence of the preceding stage. The stages are:
1. Loss of water from the aqueous layer of the tear film leading to an increase in the tear film osmolarity;
2. Loss of conjunctival goblet cells and decreased corneal glycogen;
3. Increased loss of corneal squamous epithelial cells;
4. Destabilization of the interface between the corneal surface and the tear film.
Either decreased secretion of tear film components or increased evaporation lead to increased tear film osmolarity and the following stages that lead to eventual corneal decompensation and the serious consequences of dry eye syndrome.
The adnexa of the eye may also be involved in dry eye syndrome. The adnexa of the eye include the structures surrounding the eye such as the eyelids, eye lashes, the tear drainage and tear production structures. Blepharitis commonly contributes to dry eye syndrome. Blepharitis typically results from bacterial infection of the tiny glands in the margin of the eyelid. These glands include the glands of Zeiss, Moll and Wolfring as well as the meibomian glands. Most commonly, the affected glands are the meibomian glands. In bacterial blepharitis, bacterial infection causes the meibomian glands to become plugged, and thus not be able to produce a normal lipid layer to contribute to the tear film. Some bacteria that infect the glands also secrete exotoxins that seep out of the glands into the eye and injure the corneal epithelium.
Treatments of dry eye syndrome vary depending upon the type of presentation. The most common treatment for dry eye syndrome is the use of artificial tear supplements to provide additional moisture and lubrication to the corneal surface. Artificial tear eye drops are placed on the eye by the patient. Artificial tear supplements must be used regularly and often to be effective.
Lubricant ointments may also be employed. Ointments are usually used at bedtime because they tend to be messy and blur vision. For some patients, even the use of ointments is not sufficient to provide comfort during sleep.
Tears drain from the eye through the lacrimal drainage system. Tiny openings at the nasal corner of each upper and lower eyelid are called the lacrimal puncta. The lacrimal puncta lead into ducts that drain into the nasopharynx
One treatment for dry eye syndrome is to partially or completely close one or more lacrimal puncta to reduce tear outflow into the lacrimal drainage apparatus. Traditionally, this closure was accomplished surgically or by cautery. In the last decade, however, temporary and permanent punctal occlusion plugs have been utilized.
Permanent punctal plugs are typically made from surgical silicone; temporary plugs are generally made of soluble collagen. Collagen plugs dissolve over a period of days and are helpful in diagnosis.
Punctal plugs are placed into the lacrimal puncta, or lacrimal drainage ducts. The plugs impede the outflow of tears from the eye. This approach slows the outflow of tears and retains them in the eyes longer, often relieving symptoms. Punctal plugs have the distinct advantage of being readily removable and avoid the issues of scar formation.
Blepharitis is sometimes treated by the use of antibiotic medications. Another important treatment for blepharitis is the application of warm soaks and lid scrubs. In this form of treatment, the patient applies a warm wet washcloth to the eyelids for a period of time to provide humidity, warmth and to help soften blockage of and restore flow from the meibomian glands. Lid scrubs are practiced by taking a mild, nonirritating soap and vigorously scrubbing the eyelid margins with they eyes closed, so as to massage the meibomian glands and increase production. The surfactant helps to dissolve the greasy blockage of the meibomian glands.
Patients who have severe dry eye syndrome often suffer disrupted sleep because they cannot go for longer than an hour or so without applying tear supplements to the eyes. This can lead to pronounced sleep deprivation and a consequent reduction in quality of life.
A variety of researchers have been seeking other medicinal treatments for dry eye syndrome. Largely, this research is directed at pharmaceutical efforts to increase tear production.
Despite the many treatment options available, there remains no cure for dry eye syndrome. A great many patients still have substantial and even debilitating discomfort because of dry eye syndrome. Very few treatment options exist to provide comfort for dry eye syndrome patients during sleep.